Wireless communication networks are a ubiquitous part of modern life in many areas. The inexorable trend in wireless communication development is a demand for higher data rates, to deliver a broader array of services and a richer user experience. One recent development with the promise to improve data rates and reliability is the use of multiple antennas in a transmitter and/or receiver. The use of multiple antennas on both the transmitter and receiver results in a multiple-input multiple-output (MIMO) communication channel, having the greatest performance gains over single-antenna or hybrid systems.
Wireless communication networks operate under one or more industry standards, such as WCDMA, WiMax, GMS/EDGE, UTMS/HSPA, and the like. One such standard is the Long Term Evolution (LTE), developed and promulgated by the 3rd Generation Partnership Project (3GPP). Release 10 of the LTE standard, also known as LTE Rel-10, or LTE-Advanced, supports MIMO antenna deployments and MIMO related techniques. A current working assumption in the uplink (UL) of LTE Rel-10 is the support of a spatial multiplexing mode (SU-MIMO) in the communication from a single User Equipment (UE) to the base station, or enhanced Node B (eNodeB or eNB). SU-MIMO targets high data rates in favorable channel conditions. SU-MIMO consists of the simultaneous transmission of multiple data streams on the same bandwidth, where each data stream is referred to as a layer. Multi-antenna techniques such as linear precoding are employed at the transmitter in order to differentiate the layers in the spatial domain and allow the recovery of the transmitted data at a receiver.
Another MIMO technique supported by LTE Rel-10 is MU-MIMO, where multiple UEs belonging to the same cell are completely or partly co-scheduled on the same bandwidth and time slots. Each UE in the MU-MIMO configuration may possibly transmit multiple layers, thus operating in SU-MIMO mode.
It is necessary to allow the receiver to estimate the equivalent channel associated with each transmitted layer in the cell, in order to allow detection of all the data streams. Therefore, each UE must transmit a unique reference signal (RS, or pilot signal) at least for each transmitted layer. Different types of RS are defined—including the DeModulation RS, or DMRS. The receiver is aware of which DMRS is associated with each layer, and performs estimation of the associated channel by executing a channel estimation algorithm, as known in the art. The estimated channel is then employed by the receiver in the detection process to recover the transmitted data from the received data stream.
According to the LTE Rel-10 standard, in its current status, a set of potential RS is defined, where each DMRS is uniquely defined by a cyclic shift (CS) value, with 12 CS values supported, and an orthogonal cover code (OCC), with 2 OCC values defined. In LTE Rel-8, downlink control information (DCI) format 0 for the Physical Uplink Shared Channel (PUSCH) scheduling includes a 3-bit field (nDMRS) for signaling of the CS for DMRS. To support SU-MIMO in the uplink of LTE Rel-10, multiple cyclic shifts and/or orthogonal cover codes must be signaled to the UE for DMRS multiplexing. However, it is not practical to signal multiple cyclic shift indices explicitly for all layers due to the large overhead that would be incurred. Accordingly, the working assumption for CS signaling is as follows:
Only one cyclic shift index is signaled in the corresponding DCI as in Rel-8. The mapped cyclic shift value nDMRS(2) from the signaled cyclic shift index nDMRS is used for DMRS of layer-0; the cyclic shift values for other layers are derived from nDMRS(2) according to a pre-defined rule. The table of FIG. 1 provides the working assumption for such pre-defined rule.
There are two possible OCC over the two DMRS symbols within one sub-frame (see FIG. 1). In addition to separating multiple DMRS by different CS, OCC can be signaled to the UE to provide better orthogonality among the multiplexed DMRS from different layers. The working assumption for OCC signaling in RAN1 is implicit signaling of OCC:
The implicitly assigned OCC can be derived from the signaled cyclic shift value: nDMRS(1)+nDMRS(2), where nDMRS(1) is provided by higher layers as a semi-static CS and nDMRS(2) is the signaled (dynamic) CS value in the most recent DCI for the corresponding PUSCH transmission, according to a pre-defined rule. The table of FIG. 1 provides the working assumption for such pre-defined rule. No additional bit is needed in the corresponding DCI for OCC signaling.
The working assumption for mapping from CS value to OCC is illustrated in the table of FIG. 1, where different OCC are mapped to adjacent CS values. Note that nDMRS(2) itself will only be able to signal 8 CS values: 0, 2, 3, 4, 6, 8, 9, and 10. However, nDMRS(1)+nDMRS(2) will be able to address all possible CS values.
The DMRS for each layer (also known as each virtual antenna) is constructed according to the following procedure.
First, after receiving the dynamic CS value nDMRS(2) from the corresponding Physical Downlink Control Channel (PDCCH) and the semi-static CS value nDMRS(1) from higher layers, according to the pre-defined rule depicted in Table 1, the mapped orthogonal cover code index is determined as: IOCC=f(nDMRS(1)+nDMRS(2)).
Second, the DMRS for each layer/virtual antenna can be constructed according to the rules depicted in Table 1 for each rank:
TABLE 1Layer-specific Rules for CS and OCC CalculationLayer (Virtual Antenna)DMRS in Slot 0 & 1Rank-10CS: nDMRS(2), OCC Index: IOCCTransmissionRank-20CS: nDMRS(2), OCC Index: IOCCTransmission1CS: nDMRS(2) +6, OCC Index: 1-IOCCRank-30CS: nDMRS(2), OCC Index: IOCCTransmission1CS: nDMRS(2) +3, OCC Index: 1-IOCC2CS: nDMRS(2) +6, OCC Index: IOCCRank-40CS: nDMRS(2), OCC Index: IOCCTransmission1CS: nDMRS(2) +3, OCC Index: 1-IOCC2CS: nDMRS(2) +6, OCC Index: IOCC3CS: nDMRS(2) +9, OCC Index: 1-IOCC
Note that, in Table 1, the CS values for each layer comprise the mapped dynamic CS value for layer 0, nDMRS(2), offset by a predetermined amount for each successive layer. Of these offsets, the minimum value is three (i.e., for rank-3 and rank-4 transmissions). Also, note that the OCC index is the value determined from the table in FIG. 1 and the semi-static CS value nDMRS(1) for layer 0, and then alternating to the other defined OCC value for each successive layer. Ideally, the combination maximally separates DMRS in successive layers, by a CS separation of three, and alternating OCC values.
Schemes for constructing the DMRS for multi-layer transmission, other than those in Table 1 above, are equivalently supported. For example, alternative rules for assigning the CS and OCC values for successive Layers/Virtual Antennas based on nDMRS(2) are possible.
In addition to MIMO support, 3GPP LTE Rel-10 additionally supports multi-carrier operations, also known as carrier aggregation, in order to improve spectrum allocation size and flexibility. In case of multi-carrier operation, independent data channels are modulated onto and transmitted on each of two or more carrier frequencies, known as component carriers (CC), or simply “carriers.” The allocation of uplink (UL) and downlink (DL) carriers is flexible, so it is possible to allocate a different set and number of DL and UL carriers for a certain UE.
Cross-CC scheduling is a new Rel-10 resource allocation modality where a single DL CC controls multiple UL CCs. Therefore, control information for all the controlled UL CCs may be conveyed on the same DL CC. For example, the collected ACK/NACK control messages (PHICH) referred to UL transmissions for all the UL CCs may be collected on the same DL CC. In order to allow multiplexing of different PHICH messages on the same CC, each PHICH message is defined by unique nPHICHgroup and nPHICHseq parameters, which are in turn functions of several allocation parameters including nDMRS for a given CC. Therefore, the working assumption in RAN1 is that cyclic shifts of UL DMRS are available as mechanism to avoid PHICH collisions. In particular, the working assumption on the PHICH formulas is:nPHICHgroup=(IPRB—RAlowest—index+nDMRS)mod NPHICHgroup+IPHICHNPHICHgroupnPHICHseq=(└IPRB—RAlowest—index/NPHICHgroup┘+nDMRS)mod 2NSFPHICH  (1)where the parameters IPRB—RAlowest—index, NPHICHgroup, IPHICH and NSFPHICH have the meanings defined in 3GPP TS 36.211, 36.212, and 36.213 (e.g., in V.9.0.0 of each the relevant specifications, which are all incorporated herein by reference in their entirety). That is, NSFPHICH is the spreading factor size used for PHICH modulation. IPRB—RAlowest—index is the lowest physical resource block (PRB) index in the first slot of the corresponding PUSCH transmission, NPHICHgroup is the number of PHICH groups configured by higher layers, and IPHICH is a constant whose value depends on the current time-division duplex (TDD)/frequency division duplex (FDD) configuration (for example, the value of IPHICH may depend on whether the UE is currently configured to use a particular subset of the possible TDD UL/DL configurations, such that:
      I    PHICH    =      {                            1                                                                                      for                  ⁢                                                                                                    ⁢                                                                                                  ⁢                  TDD                  ⁢                                                                          ⁢                                      UL                    /                    DL                                    ⁢                                                                          ⁢                  configuration                  ⁢                                                                                                    ⁢                                                                                                  ⁢                  0                  ⁢                                                                          ⁢                  with                  ⁢                                                                          ⁢                  PUSCH                                                                                                                          transmission                    ⁢                                                                                  ⁢                    in                    ⁢                                                                                  ⁢                    subframe                    ⁢                                                                                  ⁢                    n                                    =                                      4                    ⁢                                                                                  ⁢                    or                    ⁢                                                                                  ⁢                    9                                                                                                            0                          otherwise                      )  Additionally, according to Rel-8 assumptions, nDMRS in equation (1) is given by the latest DCI format 0.
In case of multi-codeword (CW) transmission on the same UL CC (as in the case of multi-layer transmission), an individual PHICH should be generated for each UL CW on each UL CC in the Cross-CC scheduling group.
The proposed working solution has several deficiencies. The scheduling flexibility appears to be limited in some cases of major practical interest, such as Cross-CC scheduling. Collision avoidance in PHICH signaling imposes constraints in the UL-DMRS allocation that reduce scheduling flexibility. Constraints on the allocation of UL-DMRS may lead to unnecessarily suboptimal performance in channel estimation due to poor orthogonality between DMRS of different UEs or layers. Reduced flexibility in the DMRS allocation due to PHICH signaling constraints leads to complex allocation procedures for DMRS. Finally, reduced flexibility in the scheduling due to DMRS constraints leads to complex resource allocation.